I'm looking for a mid level (4-8lvl) underwater adventure. I seem to recall some original AD&D adventure but the name escapes me. Can anyone recommend something along these lines? Could be from anyone , I'm going to convert to Castles & Crusades.

You may be thinking of the classic AD&D U (for Underwater) series:
- U1 The Sinister Secret of Saltmarsh (L1-3 above ground)
- U2 Danger at Dunwater (L1-4 mix of above ground and some underwater)
- U3 The Final Enemy (L3-6 underwater)
IIRC, these were the first major adventures to come out of the UK. They are very deserving of their classic status because, in addition to being great adventures in themselves, they featured a coherent story arc that ran through the whole series (and wasn't just an superficial prompt for the next adventure).
My D&D group remembers this series especially fondly as they made an excellent start to what turned into the longest/richest single campaign of our gaming.

The Lankhmar campaign series also had a good underwater adventure tackling the Sea King's lair. If you can find it, this one may be more suited to the level you're aiming for.

I thought I just saw a Pathfinder underwater adventure...

Dungeonland or Through the Magic Mirror had a bit of an underwater adventure, a quest from the Mock Turtle.

Pathfinder adventure "Shore to Sea" while not underwater, is considered an 'aquatic' adventure.